Mosby was born in Powhatan County, Virginia on December 6, 1833, to Virginia McLaurine Mosby and Alfred Daniel Mosby, a graduate of Hampden-Sydney College. His father was a member of an old Virginia family of English origin whose ancestor, Richard Mosby, was born in England in 1600[2] and settled in Charles City, Virginia in the early 17th century. Mosby was named after his paternal grandfather, John Singleton.

Mosby began his education at a school called Murrell's Shop. When his family moved to Albemarle County, Virginia (near Charlottesville) in about 1840, John attended school in Fry's Woods before transferring to a Charlottesville school at the age of ten. Because of his small stature and frail health, Mosby was the victim of bullies throughout his school career. Instead of becoming withdrawn and lacking in self-confidence, the boy responded by fighting back, although the editor of his memoirs recounted a statement Mosby made that he never won any fight in which he was engaged. In fact, the only time he did not lose a fight was when an adult stepped in and broke it up.[3]

In 1847, Mosby enrolled at Hampden-Sydney College, where his father was an alumnus. Unable to keep up with his mathematics class,[4] Mosby left the college after two years. On October 3, 1850,[5][6][7] he entered the University of Virginia, taking Classical Studies and joining the Washington Literary Society and Debating Union. He was far above average in Latin, Greek, and literature (all of which he enjoyed), but mathematics was still a problem for him. In his third year a quarrel erupted between Mosby and a notorious bully, George R. Turpin, a tavern keeper's son who was robust and physically impressive. When Mosby heard from a friend that Turpin had insulted him, Mosby sent Turpin a letter asking for an explanation—one of the rituals in the code of honor to which Southern gentlemen adhered. Turpin became enraged and declared that on their next meeting, he would "eat him up raw!" Mosby decided he had to meet Turpin despite the risk; to run away would be dishonorable.[8]

On March 29 the two met, Mosby having brought with him a small pepper-box pistol in the hope of dissuading Turpin from an attack. When the two met and Mosby said, "I hear you have been making assertions ..." Turpin put his head down and charged. At that point, Mosby pulled out the pistol and shot his adversary in the neck. The distraught 19-year-old Mosby went home to await his fate. He was arrested and arraigned on two charges: unlawful shooting (a misdemeanor with a maximum sentence of one year in jail and a $500 fine) and malicious shooting (a felony with a maximum sentence of 10 years in the penitentiary). After a trial that almost resulted in a hung jury, Mosby was convicted of the lesser offense but received the maximum sentence. Mosby later discovered that he had been expelled from the university before he was brought to trial.[9]

While serving time, Mosby won the friendship of his prosecutor, attorney William J. Robertson. When Mosby expressed his desire to study law, Robertson offered the use of his law library. Mosby studied law for the rest of his incarceration. Friends and family used political influence in an attempt to obtain a pardon. Gov. Joseph Johnson reviewed the evidence and pardoned Mosby on December 23, 1853. In early 1854, his fine was rescinded by the state legislature. The incident, trial, and imprisonment so traumatized Mosby that he never wrote about it in his memoirs.[10]

After studying for months in Robertson's law office, Mosby was admitted to the bar and established his own practice in nearby Howardsville. About this time, Mosby met Pauline Clarke, who was visiting from out of town. He was Methodist and she was Catholic, but their courtship ensued. Her father was an active attorney and well-connected politician. They were married in a Nashville hotel on December 30, 1857 and after living for a year with Mosby's parents, the couple settled in Bristol, Virginia which was close to Clarke's hometown in Kentucky. They had two children before the Civil War and another was born during it.[11]

Mosby spoke out against secession, but joined the Confederate army as a Private at the outbreak of the war. He first served in William "Grumble" Jones's Washington Mounted Rifles. Jones became a Major and was instructed to form a more collective "Virginia Volunteers", which he created with two mounted companies and eight companies of infantry and riflemen, including the Washington Mounted Rifles. Mosby was upset with the Virginia Volunteers' lack of congeniality, and he wrote to the governor requesting to be transferred. However, his request was not granted. The Virginia Volunteers participated in the First Battle of Bull Run (First Manassas).

In January 1863, Stuart, with Lee's concurrence, authorized Mosby to form and take command of the 43rd Battalion Virginia Cavalry. This was later expanded into Mosby's Command, a regimental-sized unit of partisan rangers operating in Northern Virginia. The 43rd Battalion operated officially as a unit of the Army of Northern Virginia, subject to the commands of Lee and Stuart, but its men (1,900 of whom served from January 1863 through April 1865) lived outside of the norms of regular army cavalrymen. The Confederate government certified special rules to govern the conduct of partisan rangers. These included sharing in the disposition of spoils of war. They had no camp duties and lived scattered among the civilian population. Mosby required proof from any volunteer that he had not deserted from the regular service, and only about 10% of his men had served previously in the Confederate Army.[13]

Having previously been promoted to captain, on March 15, 1863, and major, on March 26, 1863, in the Provisional Army of the Confederate States, Mosby was soon promoted to lieutenant colonel on January 21, 1864, and to colonel, December 7, 1864.[14]

Mosby is famous for carrying out a daring raid far inside Union lines at the Fairfax County courthouse in March 1863, where his men captured three Union officers, including Brig. Gen.Edwin H. Stoughton. Mosby wrote in his memoirs that he found Stoughton in bed and roused him with a "spank on his bare back." Upon being so rudely awakened the general indignantly asked what this meant. Mosby quickly asked if he had ever heard of "Mosby". The general replied, "Yes, have you caught him?" "I am Mosby," the Confederate ranger said. "Stuart's cavalry has possession of the Court House; be quick and dress." Mosby and his 29 men had captured a Union general, two captains, 30 enlisted men, and 58 horses without firing a shot.[15][16]

Mosby led 100 men on a raid across the Potomac River to attack the Union camp at Seneca, Maryland on June 10, 1863. After routing a company of the Sixth Michigan Calvary and burning their camp, Mosby reported to J.E.B. Stuart of the success. This drew Stuart's attention to Rowser's Ford, the point where Mosby had crossed the Potomac. This would impact the Gettysburg Campaign, as Stuart crossed Rowser's Ford the night of June 27 and was now separated from Lee's army and wouldn't arrive at Gettysburg until the afternoon of the second day of the battle. Lee stumbled into the battle without his cavalry, partly because of Mosby's successful skirmish at Seneca three weeks earlier.[17]

Mosby endured his first serious wound of the war on August 24, 1863, during a battle near Annandale, Virginia, when a bullet hit him through his thigh and side. He retired from the field with his troops and returned to action a month later.[18]

Mosby endured a second serious wound on September 14, 1864, while taunting a Union regiment by riding back and forth in front of it. A Federal bullet shattered the handle of his revolver before entering his groin. Barely staying on his horse to make his escape, he resorted to crutches during a quick recovery and returned to command three weeks later.[19]

The families of most of Moseby's men are know[n] and can be collected. I think they should be taken and kept at Fort McHenry or some secure place as hostages for good conduct of Mosby and his men. When any of them are caught with nothing to designate what they are hang them without trial.[20]

On September 22, 1864, Union forces executed six of Mosby's men who had been captured out of uniform in Front Royal, Virginia; a seventh (captured, according to Mosby's subsequent letter to Sheridan, "by a Colonel Powell on a plundering expedition into Rappahannock") was reported by Mosby to have suffered a similar fate.[21] William Thomas Overby was one of the men selected for execution on the hill in Front Royal. His captors offered to spare him if he would reveal Mosby's location, but he refused. According to reports at the time, his last words were, "My last moments are sweetened by the reflection that for every man you murder this day Mosby will take a tenfold vengeance."[22] After the executions a Union soldier pinned a piece of paper to one of the bodies that read: "This shall be the fate of all Mosby's men."[23]

After informing General Robert E. Lee and Confederate Secretary of War James A. Seddon of his intention to respond in kind, Mosby ordered seven Union prisoners, chosen by lot, to be executed in retaliation on November 6, 1864, at Rectortown, Virginia. Although seven men were duly chosen in the original "death lottery," in the end just three men were actually executed. One numbered lot fell to a drummer boy who was excused because of his age, and Mosby's men held a second drawing for a man to take his place. Then, on the way to the place of execution a prisoner recognized Masonic regalia on the uniform of Confederate Captain Montjoy, a recently inducted Freemason then returning from a raid. The condemned captive gave him a secret Masonic distress signal. Captain Montjoy substituted one of his own prisoners for his fellow Mason[25] (though one source speaks of two Masons being substituted).[26] Mosby upbraided Montjoy, stating that his command was "not a Masonic lodge". The soldiers charged with carrying out the executions of the revised group of seven successfully hanged three men. They shot two more in the head and left them for dead (remarkably, both survived). The other two condemned men managed to escape separately.[27]

On November 11, 1864, Mosby wrote to Philip Sheridan, the commander of Union forces in the Shenandoah Valley, requesting that both sides resume treating prisoners with humanity. He pointed out that he and his men had captured and returned far more of Sheridan's men than they had lost.[28] The Union side complied. With both camps treating prisoners as "prisoners of war" for the duration, there were no more executions.

Mosby had his closest brush with death on December 21, 1864, near Rector's Crossroads in Virginia. Apparently having dinner with a family in a Southern home, Mosby was fired on through a window, and the ball entered his abdomen two inches below the navel.[18] He managed to stagger into the bedroom and hide his coat, which had his only insignia of rank. The commander of the Union detachment, Maj. Douglas Frazar of the 13th New York Cavalry, entered the house and—not knowing Mosby's identity—inspected the wound and pronounced it mortal. Although left for dead, Mosby recovered and returned to the war effort once again two months later.[30]

Several weeks after General Robert E. Lee's surrender, Mosby simply disbanded his rangers on April 21, 1865, in Salem, Virginia, as he refused to surrender formally. Many of his men obtained official parole documents from the Federals and returned to their homes, but Mosby himself traveled southward with a small party of officers to join up with General Joseph E. Johnston's army in North Carolina. Before he reached his fellow Confederates, he read in a newspaper of Johnston's surrender. Some of his colleagues proposed that they return to Richmond and capture the Union officers who were occupying the White House of the Confederacy, but Mosby rejected the plan, telling them, "Too late! It would be murder and highway robbery now. We are soldiers, not highwaymen."[31]

Mosby was a wanted man, with a $5000 bounty on his head issued by Maj. Gen. Winfield S. Hancock. He eluded capture in the area of Lynchburg, Virginia, until the end of June, when Ulysses S. Grant intervened directly in the case and paroled him.[32]

After the war, Mosby became an active Republican, saying it was the best way to help the South. Mosby went on to become a campaign manager in Virginia for President Ulysses S. Grant. In his autobiography Grant stated, "Since the close of the war, I have come to know Colonel Mosby personally and somewhat intimately. He is a different man entirely from what I supposed. He is able and thoroughly honest and truthful."[33]

A major postbellum activity for Mosby was his prolific defense of J.E.B. Stuart, who had been blamed by some partisans of the Lost Cause for the Confederate failure at Gettysburg. Mosby had served under Stuart during the campaign and was fiercely loyal to the late general, writing, "He made me all that I was in the war. ... But for his friendship I would never have been heard of." He wrote numerous articles for popular publications and published a book length treatise in 1908, a work that relied on his skills as a lawyer to refute categorically all of the claims laid against Stuart. A recent comprehensive study of the Stuart controversy, written by Eric J. Wittenberg and J. David Petruzzi, called Mosby's work a tour de force.[34]

Mosby's friendship with Grant, and his work with those whom many Southerners considered the enemy, made Mosby a highly controversial figure in Virginia. He received death threats, his boyhood home was burned down, and at least one attempt was made to assassinate him. Said Mosby: "There was more vindictiveness shown to me by the Virginia people for my voting for Grant than the North showed to me for fighting four years against him."[35] The danger contributed to the President's appointing him as U.S. consul to Hong Kong (1878–1885). Mosby then served as a lawyer in San Francisco, California with the Southern Pacific Railroad. Later he worked for the Department of the Interior, first enforcing federal fencing laws in Omaha, then evicting trespassers on government-owned land in Alabama. He also worked as an assistant attorney in the Department of Justice (1904–10).[36]

Mosby was friends with the family of George S. Patton. Mosby visited the Patton Ranch and recreated Civil War battles with George, with Mosby playing himself and George playing General Lee.[37]

In a June 1907 letter to Samuel "Sam" Chapman, Mosby explained why he fought for the Confederacy, despite personally disapproving of slavery. While he admitted that the Confederacy had seceded to protect slavery, he had felt it was his patriotic duty as a Virginian to fight for the Confederacy. "I am not ashamed of having fought on the side of slavery—a soldier fights for his country—right or wrong—he is not responsible for the political merits of the course he fights in ... The South was my country."[38]

The area around Middleburg, from where Mosby launched most of his behind-the-lines activities, was called "Mosby's Confederacy", even in the Northern press. The Mosby Heritage Area Association in Middleburg, headquartered in Middleburg, is actively involved in preserving the history, culture, and scenery of this historic area.[39]